Tau is a microtubule-associated protein that plays a pivotal role in the pathogenesis of Alzheimer's disease (AD). Site-specific phosphorylation regulates tau function and in AD brain tau is abnormally phosphorylated, is functionally impaired, is abnormally cleaved and accumulates as filamentous structures, events that likely impair neuronal function. Although it is clear that all these changes in tau take place in AD brain, the sequence of events and their contribution to neuronal cell death in Alzheimer's disease has not been clearly delineated. Our long range goal is to fully elucidate the sequence of pathological processes that result in aberrant posttranslational processing of tau and how these events compromise neuronal survival in AD brain so that effective therapeutics can be developed. The objective of this project is to determine how the phosphorylation of specific sites on tau affect tau-microtubule interactions, tau oligomerization, tau interaction with molecular chaperones (and thus its functional state), and tau turnover through the ubiquitin-proteasome system. The central hypothesis of this application is that a cascade of tau phosphorylation events in which glycogen synthase kinase 3 (GSK3) plays a key role, initially results in impaired microtubule binding and subsequently, in conjunction with caspase-cleavage, results in tau aggregation. These processes disrupt the ability of chaperones to regulate tau function and turnover and ultimately result in increased cellular toxicity. The rationale for these studies is that once it is known how specific posttranslational processes negatively impact tau function and contribute to neuronal dysfunction and death, then therapeutic targets for the treatment of Alzheimer's disease can be identified. The objectives of this project will be accomplished through three Specific Aims that test the hypotheses: (1) that phosphorylation of key sites on tau impairs tau function, increases tau-tau interactions and plays a central role in tau's facilitation of cell death processes, (2) that caspase cleavage of tau increases its propensity to self-associate and decrease cell survival, and that these effects are exacerbated by phosphorylation of specific sites on tau, and (3) that chaperones and the E3 ubiquitin ligase CHIP control tau conformation and proteasome targeting, that these interactions are regulated by site-specific tau phosphorylation and that tau cleaved by caspases accumulates because it is no longer an efficient substrate of the proteasome system. Overall these are important and timely studies that will clearly define the role of site-specific phosphorylation, caspase cleavage and the impact of chaperones and the ubiquitin-proteasome system on pathological changes in tau that contribute to the disease processes in Alzheimer's disease.